Pressure indicating apparatus



. Patentecl May 18, 1.943l I yUNITED STATES PATENT oFFIcE PRESSURE INDICATIN G APPARATUS Charles S. Draper, Boston, Mass., and Joseph H. Lancor, Jr., Brooklyn, and Yao Tzu Li, New. York, N. Y., assignors to Research Corporation, New York N. Y., a corporation of New York Application March'29, 1940, Serial N0. 326,626 9 Claims. (Cl. 177-311) This invention relates to apparatus for indisponsve device which is insensitive to vibration eating the rate of change of pressure within a of its mounting.

rate of change of pressure within an engine cylin-- Still another obJect is to provide in such a deder over a wide range of operating conditions vice elastic means capable of vibration for neuincluding the condition of detonation. tralizing the eiect of cylinder vibration Detonation or knock, which is particularly Other objects and advantages of this invention apt to occur in high compression engines such as will become apparent as the description PrOCeedS. aircraft engines, is a destructive phenomenon Referring to the drawings,

characterized by a steep front, oscillatingv pres- Fig- 1 1S Sectional View 0f a Dick-11D unit fOI sure wave having component frequencies rangindicating apparatus according to one form of ing up to 25,000 cycles per second or higher. the invention, assembled on the wall of an engine Only devices capable of following these extremely cylinder or other closed chamber.

rapid changes can give a. true indication of the Fig. 2 is a partial section along the line 2-2 pressure or pressure changes which exist in the 0f Fig. 1. cylinders of a detonating engine and thereby Fig. 3 is a sectional view cf a modified form of furnish a measure of the potential danger to the pick-up unit which has Vibration compensating engine from this abnormal operating condition. Ameans and is shown on a larger scale than Fig. 1. Previous cylinder pressure indicating devices Fig. 4 is a partial section along the line 4-4 have employed structural arrangements which of Fig. 3.

A Fig. 5 is a View, partly in section, of another too low to permit their giving an accurate indimodification of our vibration compensated type cation of detonation and have been found to be 0f Dick-11D Unit.

-unstable in operation. Experience in operating Fig. 6 is a schematic diagram illustrating the instruments of this type has also shown that it wirng and arrangement of a complete electrical obtain an indication of pressure directly and the cluding amplifying, viewing and recording means.

formance, particularly detonation, since it aceld to generate an E. M. F. proportional to centuates changes which ordinarily are of more rate of change of pressure. The present ininterest than steady conditions, vention also makes use of a pressure actuated di- A further disadvantage of earlier engine presaphragm but eliminates the moving coil and sure indicating devices is overcome in certain causes the displacement of the diaphragm unfor neutralizing the eiect of vibration transmitreluctance of a magnetic circuit linked with a ted to the instrument from the engine cylinder, coll to generate an E M F therein which may be an important source 0f error, Referring particularly to Fig 1, reference nu- One obJect of the present invention is to promerel l 1S applied t0 a hOllOW CaSlI-lg 0f Steel 01 accurate measure of change of pressure in a 5o ably adapted for threaded engagement at one A further object is to provide apparatus furdisplaced inwardly or outwardly therefrom. A

construction and which is operative over a wide pressed into a recess at'the threaded end ofthe casing and is supported at its periphery by the A further object is to provide a pressure ree0 casing, the central portionmf the diaphragm be- 1 pole piece inch in a preferred design,

determine yits natural cylinder to facilitate assembly, is of such slight,

(neglecting the effect of uid loading and damping), reside wholly in said diaphragm.

In operation, a change of cylinder pressure, for example, an increased pressure, causes the central part of diaphragm 3 to be deflected and. move toward pole piece t? a distance proportional to the increase in pressure thereby increasing depth as to have no effect on the gas pressure c exerted on `the diaphragm. Therefore, for all practical purposes the diaphragm may be considered to be located ush with or form a continuation of the cylinder wall. Supported centrally magnet t, preferably of cobalt steel, axially magnetized and in contact at its upper end with an annular spacer 5 of permeable material magnetically connecting the magnet d and casing The term permeable as used in this specification and in the claims refers to material having al relatively high magnetic permeability such as iron and the ferrous alloys while the term nonpermeable refers to material having a relatively low magnetic permeability such as most of the non-ferrous metals. Y

Magnet d bears at its lower end permeable pole piece 6 which serves as a core for coil l. Pole piece t is rigidly spaced from the casing by means of annular member 8 of non-permeable material and magnet d may be surrounded by a hollow annular jackett likewise of non-permeable material having inlet and outlet pipes lll and ill', respectively, by 'means of which a flow of water or other cooling liquid may be maintained through the jacket, although in many applications liquid cooling has been found to be unnecessary.

Connection to one e d of coil 'l is by means of insulated lead ll connected to lug l2 w ich is positioned and clamped by screw le threaded into insulated bushing ill. The opposite end of coil i may be grounded to the casing (by a connection not shown) or may be brought out to a separate terminal to avoid the effects of erratic vcirculating currents in the metallic ground Diaphragm 3 is spaced from the lower end of t by a small air gap d. Since the diaphragm is directly exposed to the pressure within the cylinder it will be deflected by changes of cylinder pressure and thereby vary the length of gap which is the only gap in a magnetic circuit otherwise completed through permeable material, including diaphragm 3, casing i, spacer' 5, magnet d and pole piece 5.

Since gap ll' forms the greater part of therewithin casing l is a cylindrical permanentV luctance of the magnetic circuit just traced, it

lwill be apparent that the ux set up in this circuit by the constant magnetomotive force of magnet 4 will vary substantially directly with gap lengthand hence with the displacement of the central portion of diaphragm 3. The diaphragm itself is relatively thick and sti, having a thickness of .060 inch and a diameter of .500

resulting in a measured natural frequencyl of vibration of 95,000 cycles perv second. It will be apparent from the construction disclosed that diaphragm 3 alone resists the entire cylinder pressure exerted upon it without the aid of cooperating st iifening members and that the displaceable portion of said diaphragm constitutes the complete moving system of the instrument. mass and elasticity of the moving system, which frequency of vibration Therefore, the effective the iiux in the magnetic circuit traced above with which coil 'l is linked by virtue of its encirclement of pole piecel 6. An E. M. F. is therefor'e generated in said coil proportional to the rate of change of ux and hence proportional to the rate of change of diaphragm displacement and to rate of change of cylinder pressure.

Referring now to the arrangement of Fig. 3. there is shown a form of our invention which operates Ain a manner similar to that of the arrangement of Fig. 1 as above described and in addition' has means which render it-insensitive to cylinder vibration. In Fig. 3 there is shown a structure comprising a cylindrical hollow casing 20 of permeable material preferably adapted for threaded engagement at one end with a cylinder wall and which has aqpair of rim-stiened diaphragms 2lv and 22, diaphragm 2l being of permeable material, preferably steel, while diaphragm 22 is cf non-permeable material such as beryllium bronze. Both diaphragms are pressed into a circular recess in the lower or threaded end by a button 25 also of permeable material rigidly mounted on diaphragm 22 but free to move in recess 22'. An air gap 28 exists between button 25 and diaphragm 2l which constitutes the major part of the reluctance of the magnetic circuit otherwise closed through permeable material. Core 2d is laterally supported at its lower end by annular member 2li'.

A coil 26 surrounds core of illustration, the connections to this coil are shown as made by way of contact prongs 2l and 21' supported in insulating bushing 29 to which the end leads of the coll are electrically connected. A connector plug (not shown), having contacts adapted to mate with prongs 21, 21 and which may be guided by a cylindrical shell surrounding said prongs, may be used to provide for attachment and detachment of an electrical cable.

Dlaphragms 2l and 22 which are relatively thick and stiff are generally similar to diaphragm 3 of Fig. 1 and both diaphragms are accurately formed to have the same natural frequency of vibration which may be of the order of that of diaphragm 3.

It will be evident that in the .arrangement of Fig. 1 any vibration of cylinder wall 2 will be transmitted directly to pole piece 6 owing to the rigid nature of the structure formed by said pole piece, spacing member 8 and casing I and any component vibration directed -along the axis of the instrument Will result in relative motion between the end of pole piece 6 and diaphragm 3, the inertia of the central portion of the diaphragm causing said central portion to retain its position in space or to act as the "seismic mass of vibration measuring instruments. This relative motion varies the flux in themagnetic cir- 24 and for purposes cuit and generates an E. M. F. in coil 'l propor- Since the small flexural displacements of the tional to the velocity of the axial vibration comdiaphragm of the pressure sensitive pick-up genponent thereby giving a false indication of erate only small E. M. F.s, the output of the pressure changes. pick-up must be considerably amplied before it 'I'he arrangement of Fig. 3 overcomes the gen- 5 is capable of actuating exhibiting or recording eration of the undesired vibration E. M. F. by means. Pick-up 40 is therefore connected to a providing in non-permeable diaphragm 22 cartwo-part amplifier, the rst part, comprising rying permeable button 25, a member adapted to stages A, B and C, being a low-voltage high-gain neutralize the effect of vibration of diaphragm amplifier for stepping-up the low voltage output 2|. It will be apparent that cylinder wall vibral0 of the pick-up, while the second part, comprising unison and since gap 28 is unchanged no E. M. F. 20 designed to operate at the relatively high vbltis generated in response to cylinder vibration, ages required to actuate the oscillograph and the device being sensitive only to changes of ma es use of a triode whose output is resistance pressure. coupled to the oscillograph plates. Cathode ray The arrangement of Fig. diiers from that oscillograph 4| is supplied with power from a l shown in Fig. 3 in that compensating diaphragm 25 suitable alternating current source and may in- 22 is omitted and the elastic properties of the clude more than one cathode ray tube, as shown, disc magnet together with the effective mass of so that if desired, pick-ups may be mounted on the magnet and coil structure which form with more than one cylinder of a multi-cylinder engine COmDenSate for cylinder Wall vibration. The 30 indication obtained at the observing or recording cular disc magnet 32 clamped at its periphery which is preferably adapted to take pictures at between a shoulder on casing 30 and spacer 33 a high enough rate is radially magnetized and mounts magnet core tions of pressure occurring during detonation. A 34 of permeable material, preferably soft iron, -illm speed of 400 ft. per sec. has been used for terminating in .button 35 at. its lower end. 40 this purpose to resolve the ne structure of the .BUOII 35 iS'SpaCed from diaphragm 3| by an .rate of change of pressure curve during periods air gap 36. Coil 31, supported by coil form 38, of detonation.

is mounted on core 34. As many changes could be made in the above Disc magnet 32 being clamped only at its construction and many apparently Widely difl periphery is free to deflect at the center and the ferent embodiments of this invention could be has a natural period of vibration equal to that and not in alimitirig sense.

0f diaphragm 3|. This Vibrating Structure COm- Having described our invention, what We claim pi-i'sing magnet 32 and the parts it mounts, and desire to secure by Letters Patent is: therefore, has the same effect as compensating In apparatus of the character described, a

2 Vibrates, both diaphragm 3| and button 35 0n 55 material closing one end of said casing. an inner preventing the generation of an E M F. in rephragm and spaced from said outer diaphragm sponse to cylinder Wall vibration Diaphragm 3l by an air gap, means for producing magnetic ilux defiects under the influence of cylinder pressure in said gap Variable'in Iaccordance with the length as in the other arrangements described generthereof and a coil linked with a substantial porating an E M F in coil 31 proportional to prestion of said ux for generating an E M F in sure changes but independent of vibration By accordance with variations of said ux said outer disc magnet, a compensated indicator is obtained vibrations at the same rate when `jointly excited by simple structural means with few parts. A by vibration transmitted to said casing. cylindrical shell 39 may be Yprovided to guide a 2. Apparatus for measuring rate of change of cable attachment plug as described in connection pressure Within a cylinder of an internal comwith Fig. 3. Y 'A The schematic diagram of Fig. 6 shows a pick- 70 ing 'adapted to project inwardly through the .cylup unit which may have the structure of any inder wall and closed at its inner end by a relaspaced from said anda coil positioned stand the cylinder pressure, a second elastic diaphragm of non-permeable material supported at the magnitude of theuX in said circuit being and a coilv controlled by the-length of said gap, linked with said magnetic circuit for generating an E. WLF. in response to rate of change of flux therein. l

3. In pressure indicating. apparatus, a hollow casing having an opening, a diaphragm of magnetically permeable material closing said opening, a disc-shaped radially magnetized permanent magnet supported at its periphery by said casing, the central portion of said magnet being `free to deflect transversely, va magnetically permeable pole piece centrally mounted on said magnet and l diaphragm by a short air gap, anda coil surroundinggsaid pole piece, the magnet, pole piece and coil forming a structural assembly substantially having a natural "period of vibration equal to that of the diaphragm thereby compensating for cylinder vibration in the apparatus.

4. In pressure indicating apparatus, a hollow cylindrical casing of magnetically permeable maasiaaie gine cylinder wail, a flux carrying member resiliently supported within said casingand-spaced from the diaphragm by an' air gap, a coil linked to said flux carrying member providing the measuring. output of the pickup, ported flux carrying member and the coil linked thereto forming a structural assembly having a natural period of vibration substantially the same as that of the relatively thick diaphragm to member and mounted on theinner of which is of non-magnetic terial, arnagnetically permeable diaphragm clos supported at its periphery only within said casing, said disc being of material having a high coercive force,.a magnetic pole piece centrally mounted on said disc and spaced from said diaphragm by a short air gap and a coil surrounding said pole piece, the disc, pole piece and coil forming a structural assembly substantially having va natural period of vibration equal to that of the -diaphragm thereby compensating for cylinder vibration in the apparatus. f

5. A magnetic pick-up for measuring the rate of change of pressure Within the cylinder of an intern-al combustion engine comprising in cornbination, a hollow casing, a rela-tively thick diaphragm of magnetic material positioned to close one end of the hollow casing and disposed substantially flush with the inner surf-ace of the engine cylinder Wall, a member of magnetic material resiliently supported in said casing and spaced from said diaphragm by an air gap, said member and the support therefor having a natural period of vibration substantially the same as rthat or the relatively thick diaphragm to compensate for cylinder vibration in the pick-up, means for providing ya constant magnetomotive force for the 'pick-up, member in proportion to link a substantial portion of the varying flux carried by said member. 6. A magnetic pick-up for measuring the .rate of change of pressure Within the cylinder of an internal combustion engine comprising. in combination, a hollow casing, a relatively thick dia-` phragm of magnetic material positioned to close one end of thevhollow casing and disposed substantially ynush with the-inner surface ci the enthe nur: varying in said to the length of said gap,

ing one end of said casing, a circular elastic disc thereby compensate for cylinder vibration in the pickup. y

7 An engine detonatlon indicator Amagnetic pickup comprising in combination, a hollow casing, an inner and outer pair of substantially parallel spaced elastic discs positioned to close one end of the hollow casing, the outer of the discs being of magnetic material and being disposed substantially flush with-the inner surface of the engine cylinder wall, a nux carrying fixed member Situated within the hollow casing, a second flux carrying member slidably engaging the fixed the parallel spaced elastic discs material,l said inner disc supporting said second iiux carry-ing member in air gap spaced relation to the outer disc,- the .respective discs havingw similar natural periods of vibration to compensate for cylinder vibration inthe pickup, and a coil linked to the nux carrying fixed member in which the flux varies in proportion-to the length of the air gap.A

8. A pick-up adapted for attachment to the wall of a cylinder of an internal combustion engine as a closure of an opening therein'ior ob- -taining an indication of the rate of change of gas pressure Within the cylinder comprising a housing, a first displaceable member supported from. said housing for exposure to the gases within the cylinder, displacement of said member by virtue of its mounting being caused `to be responsive jointly to change gas pressure and displacementof the housing, a second displaceable member supported from said housing in spaced relationship to said i'lrst member and arranged to avoid exposure tothe cylinder gases, displacement of said second member being thereby caused to be responsive substantially solely to displacemen-t of said housing and means for measuring I relative displacement bf said two members including magnetic means for producing i'l-ux vari-l able in accordance with the spacing of said memfor translating variation of said bers and means flux into an electrical quantity.

9. A pick-up for indicating change of gas pressure in the cylinder of an internal combustion engine comprising members arranged to form a magnetic circuit including ashort air gap, said gap being bounded on one side by ya member arranged for displacement in accordance with cyl;s inder wall vibration and in accordance with change of gas pressure within the cylinder to which the pick-up is attached, said gap being bounded ron the other side by a member arranged for displacement in accordance with cylinder w-all vibration unmodified by change of gas pressure, a source of magneto-motive force for setting up magnetic iiux in said circuit the amount of said nur; being determined by the length of said gap, and indicating means actuated in accordance with change of said flux.

. CHARLES S, DRAPER.

JOSEPH H. LANCOR, Jn. YAO TZU LI.

said resiliently sup- 

